Conventional helicopters, while offering drastically improved manoeuvrability over airplanes, are limited to travelling at relatively low speeds. During vertical motion or hovering, the helicopter is oriented horizontally such that the main rotor is driven for rotation about a generally vertical axis to create lift. To achieve forward motion, extra power is applied, collective pitch is increased and the helicopter is tilted nose down out of the horizontal orientation by adjusting the cyclic pitch to increase the angle of attack of the rotor blades during a portion of their rotation in which they extend rearward from the hub, thereby create more lift near the rear of the aircraft. With the aircraft in this tilted position, the rotor acts to create both lift and forward thrust.
During forward flight, the effective air speed of a blade as it advances in its rotation is the sum of the forward speed of the helicopter and the blades rotational speed, as the motion of the blade relative to the helicopter is in a forward direction. The effective air speed of a retreating blade however, is the difference between the rotational speed and the forward speed of the helicopter, as they are in opposite directions. Since lift varies with the square of velocity, the advancing blade will thus produce more lift than the retreating blade. This dissymmetry of lift can be counteracted by flapping and cyclic feathering of the blades, which increase and decrease the angles of attack of the retreating and advancing blades respectively during forward flight to create a balance of lift between the two sides. Increasing the angle of attack too much will cause a blade to stall, as smooth laminar airflow over the surfaces of the blade is lost. As the critical angle of attack is approached, the blades undergo violent vibrations known as buffeting. As a result, conventional helicopters are limited in their maximum speed as increasing the forward velocity leads to a need for increased angle of attack for retreating blades, and a high angle of attack will lead to stalling and a corresponding lack of lift.
Compound helicopters have been developed to try and overcome the speed limitations of conventional helicopters. These compound aircraft combine features of the helicopter with those of an airplane in an attempt to provide the manoeuvrability of the former and the speed of the latter. U.S. Pat. Nos. 2,531,976 and 2,575,886 by Garrett and Myers respectively and U.S. Patent Application Publication Number 2005/0151001 by Loper describe compound helicopters that have wings and nose mounted propellers that provide lift and thrust respectively for forward flight at speeds that could not be achieved using their main rotors. Garrett teaches a main rotor assembly that is folded down into a fuselage of the aircraft during forward flight. Myers teaches a main rotor that is stopped in a position parallel to the line of flight when approaching the stalling speed so as not to create drag during forward flight provided by the propeller and wings. Loper teaches a main rotor that is unloaded to autogyrate during cruising flight so that the majority of lift is provided by the wings. The presence of wings on opposite sides of these aircraft may decrease the efficiency of using the main rotor to create lift during vertical movement, hovering and the transition from hovering to forward flight as their surface area creates vertical drag. Wings also increase the weight of the aircraft and the cost of its manufacture due to more material and assembly requirements. The wings of a compound helicopter do not automatically eliminate all rotor related issues in forward flight. For example, in compound helicopters with the rotor arranged to free-wheel in forward cruising, the weight and drag created by the free-wheeling rotor still have to be dealt with.
As a result, there is a desire for a rotor-based aircraft capable of higher forward cruising speeds than a conventional helicopter without requiring the addition of wings below the main rotor on each side of the fuselage.